This invention relates to a process for modifying a shaped polyethylene and more particularly to an improved process for cross-linking a shaped polyethylene by irradiating a sheet or hollow tube of polyethylene with electron beams to thereby improve the electrical, mechanical, thermal and other characteristics of the polyethylene.
2. Description of the Prior Art
It is known that polyethylene irradiated with an electron beam is cross-linked and has its mechanical, chemical and thermal characteristics improved significantly. Among techniques which have been used for such purpose on a commercial scale include one of irradiating a hollow tube of polyethylene with electron beams to render it highly heat resistant and of irradiating a thin film of polyethylene with electron beams to provide a heat resistant formed sheet. An electron beam accelerator is generally used to treat such shaped polyethylenes with an ionizing radiation because it provides a large dose of ionizing radiation relatively easily and requires only a short period of irradiation to improve the mechanical and thermal properties of the polyethylene. An accelerator generating electrons of higher energy is advantageous because it can be used to treat materials of various thicknesses, but because of its high price, accelerators generating electrons of low energy have been practically used and attempts have been made to its limited scope of application by making modifications of the way in which it is used. For instance, if the maximum range of penetration of an electron beam is smaller than the maximum thickness of rubber or plastics which is to be treated with such beams, a single irradiation in one direction cannot cover the entire thickness of the shaped article, and therefore, the article is first irradiated on one side and then turned over for another irradiation on the opposite side. However, this technique of irradiating rubber or shaped plastics of relatively large thickness has the disadvantage of developing hair cracks in the treated article and accordingly its electrical and mechanical characteristics are degraded. Such cracks presumably develop because electrons implanted by the irradiation gradually accumulate in the irradiated article, but their distribution or effects of the electric field formed has not been completely unravelled, nor has any method been established to prevent the formation of cracks. For this reason, irradiation of thick rubber or shaped plastics with an electron beam of a maximum penetration range smaller than the maximum thickness of the shaped article has been considered inadvisable, and as a result, the technique of using electron beams to modify rubber or shaped plastics has had a limited scope of application.
Therefore, development of a technique for irradiating rubber or shaped plastics with electron beams having a maximum thickness less than the maximum thickness of the shaped article has been considered indispensable to expanding the scope of use of the method of cross-linking a shaped article with electron beams.